113 Cd-substituted human and bovine erythrocyte carbonic anhydrases have been studied by 113Cd nmr as a function of pH and bicarbonate concentration. Plots of chemical shifts versus pH give sigmoidal titration curves in the pH range of the study, 6.9 to 10.5. The pKa values vary from 9.2 to 9.7, which correlates well with available activity profiles for the Cd-enzymes. The samples contain no buffers and no anions other than hydroxide. The results point to the existence of high and low pH forms of the enzymes in rapid exchange, differing only in inner sphere coordination. When bicarbonate is added to the samples, upfield shifts are produced which eventually level off. Only a single Cn negative ion binds to the metal for each of the enzymes. These observations are best explained by a rapid exchange among three species in which the open coordination site of the metal ion is occupied by hydroxide, water or bicarbonate.